Hyperbolic field mass filter

ABSTRACT

A hyperbolic field mass filter (quadrupole or monopole) is described which includes at least two primary electrodes, an ion entrance aperture, an ion exit aperture and a.c. and d.c. bias sources coupled to the primary electrodes for producing a.c. and d.c. field components between the electrodes. At least one auxiliary electrode is disposed between the primary electrode and the entrance aperture together with means for biasing the auxiliary electrode with d.c. potentials to neutralize the d.c. field produced by the primary electrodes in the vicinity of the entrance aperture.

United States Patent [191 Brubaker HYPERBOLIC FIELD MASS FILTER {76]inventor: Wilson M. Brubaker, 1954 Highland Oaks Dr., Arcadia, Calif.91006 22 Filed: Mar. 3, 1971 21 Appl.No.: 120,482

52 U.S. c1 .Q ..250/292 [51] Int. Cl. 1 H0lj 39/34 [58] Field of Search250/419 DS [56] References Cited UNlTED STATES PATENTS 1129,32? 4/1964Brubaker 250/419 3,371,204 2/1968 Brubaker 250 419 Jan. 1,1974

Primary ExaminerWilliam F. Lindquist Attorney-Jackson & Jones 5 7ABSTRACT A hyperbolic field mass filter (quadrupole or monopole) isdescribed which includes at least two primary electrodes, an ionentrance aperture, an ion exit aperture and a.c. and d.c. bias sourcescoupled to the primary electrodes for producing a.c. and d.c. fieldcomponents between the electrodes. At least one auxiliary electrode isdisposed between the primary electrode and the entrance aperturetogether with means for biasing the auxiliary electrode with d.c.potentials to neutralize the d.c. field produced by the primaryelectrodes in the vicinity of the entrance aperture.

9 Claims, 4 Drawing Figures Arm/M Amy/r IIYPERBOLIC FIELD MASS FILTERBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates 'to quadrupole and monopole mass filters and particularly to thedisposition of auxiliary electrodes adjacent to the entrance aperture ofthe filter and the application of bias potential thereto to provide adelayed d.c. ramp mode of operation.

2. Description of the Prior Art Quadrupole and monopole mass filterssort out a beam of ions into a spectrum according to the mass (mass tocharge ratio) of the incident ions. These mass analyzers arecharacterized by their utilization of combined a.c. and dc. fields, ascontrasted to the use of magnets by most of the earlier mass analyzertypes.

A quadrupole mass filter includes four primary electrodes in the form ofparallelcylindrical rods arranged symmetrically about a central axis ofthe filter. Alternating and direct current voltages are applied in equalbut opposite values to neighboring electrodes. The a.c. voltagecomponent is in the radio frequency (r.f.) range.

The theory of operation of the quadrupole mass filter is set forth inthe US. Pat. No. 3,129,327 which was granted to me in 1964. A summary ofthe theory is presented here. The force equations for the motion of 7ions in the combined a.c. and dc. fields are readily transformed to thestandard form of the Mathieu Equation. Solutions of the Mathieu Equationreveal regions of stability and those. of instability of thetrajectories of the ions. The stability diagram as illustrated in FIG. 1is divided into alternate bands of stable and unstable portions. Theordinate axis is the dimensionless vari-' able a, while the abscissa isthe dimensionless variable q. These variables are defined in FIG. 1. Fora given a.c. and dc. excitation level, the loci of the working pointsfor all masses fall on' the scan line.

In the ion source the working point is at the origin, since thepotentials of the quadrupole electrodes do not penetrate this region. Asthe'transmitted ion passes I from the ion source into the prior artfilter through the fringe field of increasing strength, the associatedworking point moves from the origin to the vicinity of the apex alongthe scan line. In so doing it penetrates to a.c. field ratio near theion entrance aperture which -is small as compared to the ratio in theuniform field region in the center of the filter. Under these conditionsthe working point moves from the origin to the apex of the trianglealong a preferred path which lies entirely within the stable portion ofthe diagram. Thus the undesired y-directed impulse is avoided andgreatly improved performance of the instrument is obtained.

In the prior art the delayed d.c. ramp has been obtained by the use ofan auxiliary set of electrodes which are energized with large a.c.potentials and nearly zero d.c. potentials. This circumstance requiresthe application of high frequency (r.f.) potentials to these electrodeswhich are at a negligible d.c. potential. This has been accomplished byusing additional high voltage r.f. feedthroughs which pierce the'vacuumwall or by the use of resistor and capacitor networks located within thevacuum wall. Either of these expedients adds to the complication of theinstrument and increases the a.c. power required to energize it.

SUMMARY OF'TI-IE INVENTION I have discovered a new and simple structurefor providing the desired delay-dc. ramp mode of operation forquadrupole or monopole mass filters. In accordance with my invention,the mass filter includes entrance and exit apertures, at least twoparallel primary electrodes and bias means for applying do. and ac.voltages to such electrodes to produce a hyperbolic electric fieldbetween the electrodes. At. least one auxiliary electrode is disposedadjacent to the entrance aperture and bias means are provided forenergizing the auxiliary electrodes with do. potentials to neutralizethe dc. field produced by the primary electrode in the vicinity of theentrance aperture.

The delayed d.c.ramp is obtained by applying d.c. potentials only to theauxiliary electrode or electrodes. The polarities of these potentialsare opposite that of the corresponding primary electrodes. The fieldresulting from the'potentials applied to the auxiliary electrode cancelsor neutralizes the dc. component of the field from the primaryelectrodes :in the vicinity of the entrance aperture. The a.c. fieldfromthe primary electrodes is of essentially normal strength in thevicinity of the entrance aperture. Thus the normalized d.c. field isBRIEF DESCRIPTION'OF THE DRAWINGS FIG. 1 is a stability diagramillustrating the operating characteristics of a quadrupole ormonopolemass filter; 7

FIG. 2 is a schematic sectional elevation of a quadrupole mass filterand a circuit diagram for providing the bias voltages in accordance withmy invention;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2; and

FIG. 4 is a schematic end view of a monopole mass filter.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 2 and 3,the quadrupole'mass filter includes a metallic housing. 10 whichencloses four primary electrodes 12, 14, 16 and 18. Electrodes 12 and 16are designated the X electrodes and electrodes l4 and 18 are designatedthe Y electrodes. The electrodes are suitably supported by highimpedance insulators 19. The central axis of the filter is designated byas is illustrated in FIG. 2.

The primary electrodes are in the form of cylindrical conducting rodsextending parallel to one another and disposed symmetrically about thecentral axis. The X electrodes lie with their centers in the X-Z plane,and the Y electrodes lie with their centers on the Y-Z plane. Ideally,the electrodes should provide a hyperbolic curvature. However, inpractice, a circular curvature is a reasonably satisfactoryapproximation.

An entrance aperture plate 20, defining an ion entrance opening 22centered on the Z axis is secured to one end of the housing by suitablemetal screws. Ions of the substance to be analyzed are furnished by anion source 24 which is secured to the plate 20 by a suitable means. Thesource 24 is connected to a suitable power supply (not shown) by a cable26.

An exit aperture plate 28 is located at the end of the housing remotefrom the entrance plate 20. The plate 28 defines an ion exit aperture 30centered on the Z axis. A rear wall 32 closes the housing and carries anion collector 34 supported on a feedthrough insulator 36.

Ions of the selected mass-to-charge ratio enter the filter through theentrance aperture 22, traverse the filter and impinge on the collector34. The ion current may be measured by a conventional measuring circuit38. The conductive housing 10 and plates 20, 28 and 32 are all at groundpotential. The interior of the housing is maintained at a low pressureby means of a suitable vacuum pump (not shown).

Four auxiliary electrodes 42, 44, 46 and 48 are positioned symmetricallyabout the filter-axis Z adjacent the entrance aperture and between aplane defined by the ends of the primary electrodes (adjacent theentrance aperture) and the ion source 24, as shown in FIG. 2. Theauxiliary electrodes 42, 44, 46 and 48 are mounted on the entranceaperture plate by means of suitable insulators 55. As is shown, the ionentrance plate 20 is provided with four cavities or recessed portionsadjacent the ion entrance aperture 22. The auxiliary electrodes 4248 arepositioned within individual cavities so that the inner surface of theauxiliary electrodes (facing the interior of the mass filter) aresubstantially flush with the inner surface 49 of the entrance plate 20.i

Voltage sources 60 and 62 provide a.c. and d.c. potential, respectively,for the primary and auxiliary electrodes as is illustrated in thecircuit diagram of FIG. 1. Diametrically opposed electrodes 12 and 16are connected via a common lead 66 to the bias sources 60 and 62 througha transformer 64. The primary electrodes 14 and 18 are connected to thebias sources by means of a lead 68. The a.c. and d.c. bias sources areshown as being provided with a ganged control arm 70 so that themagnitude of the d.c. potential and the amplitude of the a.c. bias maybe varied together to cause ions of a selected mass to be transmittedthrough the filter to the collector. It should be noted that the gangedcontrol of the a.c. and d.c. bias sources is used for illustration only.The d.c. potential would normally be obtained from the a.c. source 60 byrectification, and would be proportional to the a.c. potential. Suchmeans are described and illustrated in my US. Pat. No. 3,129,327. As isillustrated in FIG. 3, the X electrodes (12 and 16) are provided with apositive d.c. potential and the Y electrodes (14 and 18) are providedwith a negative d.c. potential.

The auxiliary electrodes 42-48 are connected to the d.c. source 62 bymeans of suitable leads and potentiometers 72 and 74. A pair of r.f.decoupling capacitors 76 and 78 are connected between opposite terminalsof the d.c. source 62 and ground to provide an effective ground for anya.c. signals from the a.c. source 60. Diametrically opposed auxiliaryelectrodes 42 and 46 are electrically connected as a pair to the movabletap of the potentiometer 74 via lead 75 for receiving a negative d.c.bias. Auxiliary electrodes 44 and 48 are connected as a pair to themovable tap of potentiometer 72 via lead 77 to receive a positive d.c.bias. The corresponding primary and auxiliary electrodes (12 and 42,etc.) are thus provided with d.c. bias voltages of opposite polarity.The taps on potentiometers 72 and 74 are adjusted to produce a d.c.field in the vicinity of the entrance aperture 22 which neutralizes orcancels out the d.c. field produced by the primary electrodes 12-18 sothat the d.c. field is substantially zero in the vicinity of theentrance aperture 22.

The a.c. fields in the vicinity of the entrance aperture are relativelyunaffected by the presence of the auxiliary electrodes. Hence, ionsentering the filter respond to the a.c. fields promptly, and to the d.c.fields, only after they have penetrated more deeply into the filter.

As is illustrated in FIG. 1, for low values of the a.c. field (near theentrance aperture) even a very small d.c. field will cause thetrajectories to be unstable in the Y-Z plane. To increase the stabilityin the Y-Z plane, I have provided a reverse polarity d.c. bias on theauxil' iary electrodes to reduce the d.c. field near the entranceaperture to substantially zero. In this manner I have achieved a highlyefficient quadrupole mass analyzer without the need for applying a.c.voltages to the auxiliary electrodes to insure that the ions aresubjected to a large a.c. component immediately upon entering theanalyzer. This structure eliminates the need for high frequency leads orinsulators for the auxiliary electrodes. Also the auxiliary electrodesmay be conveniently mounted within the cavities of the entrance apertureplate 20 with the bias leads 75 and 77 being incorporated into the powersupply cable for the ion source.

By mounting auxiliary electrodes 40-48 so that their inner surfaces areflush with the inner surface of the entrance plate 20, the auxiliaryelectrodes do not shield the region of the entrance aperture from thea.c. fields produced by the primary electrodes. This manner of mountingthe auxiliary electrodes provides a structure which is simple andinexpensive to manufacture. The auxiliary electrodes are carried by theentrance plate 20 and the ion source and the connecting leads for theelectrodes may be integrated into the leads which energize the ionsource.

A monopole mass filter is illustrated in FIG. 4 and includes primaryelectrodes 14' and 80. Electrically the monopole mass filter is exactlyone-fourth of a quadrupole filter. It was observed by Ulf Von Zahn andreported in Review of Scientific Instruments, Vol. 34, pages 1-4, 1963that the planes of symmetry in a quad rupole mass filter appear midwaybetween the primary electrodes and that these planes of symmetry areequipotential surfaces. The replacement of three of the primaryelectrodes with a conducting surface in the form of a 90 angle plate 80as shown in FIG. 4 provides the same type of hyperbolic field as thatprovided in the quadrupole mass filter illustrated in FIG. 2. Theprimary electrode 14 is connected to suitable a.c. and d.c. potentialsources. Such sources are illustrated in FIG. 4. The electrode 80 isconnected to ground. This geometry adds an additional constraint tothose of the normal quadrupole mass filter in thations can cross theinstrument axis (the apex of the electrode 80) only at the extremitiesof the filter. Under these conditions the loci of the working points ofthe transmitted ions lie within a narrow band slightly displaced fromthe Y- stability limit. The axial velocity of the ions becomes animportant factor in determining the transmission of the ions through thefilter, and the ratio of d.c. to a.c. potentials is of lesserimportance. The use of an auxiliary electrode 44 which is energized witha d.c. voltage of opposite polarity to the d.c. voltage applied to theelectrode 14' and of sufficient magnitude to neutralize the d.c. fieldadjacent the entrance aperture permits the monopole filter .to alsooperate in a delayed d.c. ramp mode. The use of an auxiliary electrodeand an appropriate bias source to neutralize the d.c. field adjacent theentrance aperture provides the same advantages for a monopole as havebeen described above for quadrupole filters.

My improved mass filter provides the desired delayed d.c. ramp mode ofoperation by means of an inexpensive and simple structure. The filteroperates at high ef ficiency as a result of a reduction in the a.c.excitation power required compared to that of the prior art systems ofachieving the delayed d.c. ramp mode of operation. i

What is claimed is:

1. In a hyperbolic field mass spectrometer including an ion source, anion detector, means defining ion entrance and exit apertures alignedwith a central axis of the spectrometer and positioned adjacent the ionsource and detector, respectively, at least two primary electrodesextending parallel to said axis, the ends of said primary electrodesdefining a plane substantially normal to said axis adjacent the entranceaperture, at least one of the primary electrodes being in the form of arod and means for applying d.c. and a.c. voltages between the primaryelectrodes to produce a hyperbolic electric field therebetween, theimprovement which comprises:

a. at least one auxiliary electrode, each auxiliary electrode having amajor portion thereof positioned between said plane defined by the endsof the primary electrodes and the ion source so that the a.c. field inthe vicinity of the ion entrance aperture produced by the a.c. voltageson the primary electrodes is substantially unaffected by the presence ofthe auxiliary electrodes, and

b. means including a d.c. voltage source for applying a potential toeach auxiliary electrode to neutralize the d.c. field produced by theprimary electrode in the vicinity of the entrance aperture.

2. The combination as defined in claim 1 including two pairs ofdiametrically opposed parallel primary rod electrodes symmetricallyspaced about said axis and at least two 'pair of auxiliary electrodes.

3. The combination as defined in claim 1 wherein the spectrometerincludes an entrance plate forming the entrance aperture, the ion sourcebeing carried by the entrance plate, the entrance plate defining acavity adjacent the entrance aperture and means for mounting theauxiliary electrode within said cavity so that the surfaces of theauxiliary electrode and the aperture plate adjacent the primaryelectrode are substantially coincident, the mounting means includingmeans for electrically insulating the auxiliary electrode from theentrance plate.

4. The combination as defined in claim 3 including two pairs ofdiametrically opposed. parallel primary rod electrodes symmetricallyspaced about said axis and two pairs of diametrically opposed auxiliaryelectrodes, an auxiliary electrode being associated with each primaryelectrode and means for energizing the auxiliary electrodes toneutralize the d.c. field component produced by the primary electrodesin the vicinity of the aperture.

5. In a multi-pole mass spectrometer the combination which comprises:

a. a housing having an ion entrance plate at one end which defines anion entrance aperture and an ion exit plate at the other end whichdefines an ion exit aperture,

b. two pairs of diametrically opposed parallel primary rod electrodesspaced symmetrically about acentral axis which extends from the entranceto the exit apertures, the ends of the primary electrodes defining aplane substantially normal to said axis adjacent the entrance aperture,

c. bias means for applying d.c. and r.f. voltages to the primaryelectrodes to produce a multi-pole electric field between the primaryelectrodes,

d. an ion source carried by the entrance aperture plate for injectingions into the spectrometer,

e. an ion detector positioned adjacent the ion exit aperture fordetecting ions,

f. at least two pair of auxiliary electrodes secured adjacent the ionentrance aperture plate and electrically insulated therefrom, each ofthe auxiliary electrodes having a major portion thereof positionedbetween said plane defined by the ends of the primary electrodes and theion source so that the a.c. field in the vicinity of the ion entranceaperture produced by the r.f. voltages on the primary electrodes issubstantially unaffected by the presence of the auxiliary electrodes,and

f. means for applying a d.c. potential to the auxiliary electrodeswhichis of opposite polarity to the d.c. voltage applied to thecorresponding adjacent primary electrodes to neutralize the d.c. fieldcomponent produced by the primary electrodes in the vicinity of theentrance aperture.

6. The combination as defined in claim 5 wherein the auxiliaryelectrodes are positioned between the plane defined by the ends of theprimary electrodes and the ion source.

7. The combination as defined in claim 5 wherein the auxiliaryelectrodes comprise two pairs of diametrically opposed electrodes, eachof the auxiliary electrodes being associated with one of the primaryelectrodes.

8. The combination as defined in claim 7 including a capacitor connectedbetween each pair of auxiliary electrodes and ground to provide a lowimpedance path for the r.f. voltages.

9. The combination as defined in claim 7 wherein the entrance plate hasan inner surface adjacent the primary electrodes, the entrance platedefining at least one cavity adjacent the entrance aperture forreceiving the auxiliary electrodes and means for mounting the auxiliaryelectrodes on the entrance plate so that the inner surface of theentrance plate is substantially coincident with the surface of theauxiliary electrodes adjacent the primary electrodesto prevent theauxiliary electrode from shielding-the entrance aperture from the a.c.fields produced by the primary electrodes, the mounting means includingmeans for electrically insulating the auxiliary electrodes from theentrance plate. a

1. In a hyperbolic field mass spectrometer including an ion source, anion detector, means defining ion entrance and exit apertures alignedwith a central axis of the spectrometer and positioned adjacent the ionsource and detector, respectively, at least two primary electrodesextending parallel to said axis, the ends of said primary electrodesdefining a plane substantially normal to said axis adjacent the entranceaperture, at least one of the primary electrodes being in the form of arod and means for applying d.c. and a.c. voltages between the primaryelectrodes to produce a hyperbolic electric field therebetween, theimprovement which comprises: a. at least one auxiliary electrode, eachauxiliary electrode having a major portion thereof positioned betweensaid plane defined by the ends of the primary electrodes and the ionsource so that the a.c. field in the vicinity of the ion entranceaperture produced by the a.c. voltages on the primary electrodes issubstantially unaffected by the presence of the auxiliary electrodes,and b. means including a d.c. voltage source for applying a potential toeach auxiliary electrode to neutralize the d.c. field produced by theprimary electrode in the vicinity of the entrance aperture.
 2. Thecombination as defined in claim 1 including two pairs of diametricallyopposed parallel primary rod electrodes symmetrically spaced about saidaxis and at least two pair of auxiliary electrodes.
 3. The combinationas defined in claim 1 wherein the spectrometer includes an entranceplate forming the entrance aperture, the ion source being carried by theentrance plate, the entrance plate defining a cavity adjacent theentrance aperture and means for mounting the auxiliary electrode withinsaid cavity so that the surfaces of the auxiliary electrode and theaperture plate adjacent the primary electrode are substantiallycoincident, the mounting means including means for electricallyinsulating the auxiliary electrode from the entrance plate.
 4. Thecombination as defined in claim 3 including two pairs of diametricallyopposed parallel primary rod electrodes symmetrically spaced about saidaxis and two pairs of diametrically opposed auxiliary electrodes, anauxiliary electrode being associated with each primary electrode andmeans for energizing the auxiliary electrodes to neutralize the d.c.field component produced by the primary electrodes in the vicinity ofthe aperture.
 5. In a multi-pole mass spectrometer the combination whichcomprises: a. a housing having an ion entrance plate at one end whichdefines an ion entrance aperture and an ion exit plate at the other endwhich defines an ion exit aperture, b. two pairs of diametricallyopposed parallel primary rod electrodes spaced symmetrically about acentral axis which extends from the entrance to the exit apertures, theends of the primary electrodes defining a plane substantially normal tosaid axis adjacent the entrance aperture, c. bias means for applyingd.c. and r.f. voltages to the primary electrodes to produce a multi-poleelectric field between the primary electrodes, d. an ion source carriedby the entrance aperture plate for injecting ions into the spectrometer,e. an ion detector positioned adjacent the ion exit aperture fordetecting ions, f. at least two pair of auxiliary electrodes securedadjacent the ion entrance aperture plate and electrically insulatedtherefrom, each of the auxiliary electrodes having a major portionthereof positioned between said plane defined by the ends of the primaryelectrodes and the ion source so that the a.c. field in the vicinity ofthe ion entrance aperture produced by the r.f. voltages on the primaryelectrodes is substantially unaffected by the presence of the auxiliaryelectrodes, and F. means for applying a d.c. potential to the auxiliaryelectrodes which is of opposite polarity to the d.c. voltage applied tothe corresponding adjacent primary electrodes to neutralize the d.c.field component produced by the primary electrodes in the vicinity ofthe entrance aperture.
 6. The combination as defined in claim 5 whereinthe auxiliary electrodes are positioned between the plane defined by theends of the primary electrodes and the ion source.
 7. The combination asdefined in claim 5 wherein the auxiliary electrodes comprise two pairsof diametrically opposed electrodes, each of the auxiliary electrodesbeing associated with one of the primary electrodes.
 8. The combinationas defined in claim 7 including a capacitor connected between each pairof auxiliary electrodes and ground to provide a low impedance path forthe r.f. voltages.
 9. The combination as defined in claim 7 wherein theentrance plate has an inner surface adjacent the primary electrodes, theentrance plate defining at least one cavity adjacent the entranceaperture for receiving the auxiliary electrodes and means for mountingthe auxiliary electrodes on the entrance plate so that the inner surfaceof the entrance plate is substantially coincident with the surface ofthe auxiliary electrodes adjacent the primary electrodes to prevent theauxiliary electrode from shielding the entrance aperture from the a.c.fields produced by the primary electrodes, the mounting means includingmeans for electrically insulating the auxiliary electrodes from theentrance plate.